This is what I understand from Genesis.
KJV
Gen 1:5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
YLT
Gen 1:5 and God calleth to the light `Day,' and to the darkness He hath called `Night;' and there is an evening, and there is a morning--day one.
The time of darkness is night.
The time of light is day.
Together they make 1 day. or Yom
That is how I see that God defines a day and night - that together make 1 day.
yôm
yome
From an unused root meaning to be hot; a day (as the warm hours), whether literally (from sunrise to sunset, or from one sunset to the next), or figuratively (a space of time defined by an associated term), (often used adverbially): - age, + always, +
Here thus we +always, - age , would read like age day
So when a number is add in the text like 1 day it defines what day in that context means.
So light and dark make 1 day = 24 hours
When Jesus says 3 days and 3 nights that would equal 3 lights and 3 darks
That is certainly the way I see it now, but that is not what I used to believe. I think much of the confusion comes from the difference between trying to uncover for ourselves what is biblically true, versus defending what we've been taught. We also have a tendency to over-value non salvational issues. Especially if we believe the Holy Spirit makes us personally infallible in matters of doctrine.